Transmission regulation



F 11, 1941- R. w. CHESNUT TRANSMISSION REGULATION Filed June 21, 1939 Rmm 2mm R l 5 RR U a V E g f P 9 2 m 7 M R N a. m M U A 2 M '3 m G a n H?CARR/ER TERMINAL CTS.

CARR/ER TERMINAL CTS,

- TERMINAL AME THE PM IS TOR THERMISTOR a 7 /N VENTOR R. W CHESNU 7M A TTORNE V Patented Feb. 11, 1941 I UNITED STATES PATENT OFFICETRANSMISSION REGULATION Application June 21, 1939, Serial No. 280,291

26 Claims.

This invention relates to electrical signaling systems and moreparticularly to methods and means for controlling the transmissioncharacteristics of such systems.

An object of the invention is to provide new and improved methods andmeans for controlling the amplification characteristics of an electricalwave amplifier. Another object is to effect automatic, continuous andprecise control of the amplification characteristics of signal repeatersin an electrical signaling system. Still another object is to facilitatethe generation of a pilot or control wave, the intensity of whichfluctuates in such relation to the fluctuations of a signal wave thatthe average intensity of the two waves combined remains substantiallyconstant or follows some predetermined law of variation. A moreparticular object of the invention is to provide new and improvedmethods and means for automatically controlling the gain of repeateramplifiers in a. wire line carrier current signaling system tocompensate for variations in the attenuation of the line.

A feature of the invention is that the gaincontrolling wave is generatedby or within the 25 signaling circuit or system to be controlled. An-

other feature is that the signaling circuit serves to modulate thecontrol wave.

The present invention is especially adapted for automatic control of theamplification afforded by repeaters in a wire line carrier telephonesystem and it is principally in terms of its embodiment in such a systemthat the invention is hereinafter described. It will be appreciated bythose skilled in the art, however, that the invention and its underlyingnovel principles are capable of embodiment in other systems and for theperformance of other functions. These broader aspects of the inventionare indicated in the appended claims.

Heretofore it has been proposed that to effect automatic control of thegain of signal repeaters in a wire line transmission system, a pilotwave be applied to the line along with the signals and caused to vary inintensity in such manner that the average intensity of signals and pilotwave combined is maintained substantially constant. Each repeater isthen made so self-adjusting under the control of its output currentsthat the average intensity of the amplified signal and pilot wavecombined is maintained at its initial or some other predetermined level,whereby a change in the transmission equivalent of the lineautomatically produces a compensating change in the gain of therepeater.

In accordance with the present invention as embodied in preferred formin a system of the kind last described, oscillations of the desiredpilot wave frequency are established in a circuit comprising signalamplifiers each of which has selfadiusting means for maintainingconstant the total power output of signals and pilot wave combined. 5The aforesaid amplifiers may be the signal amplifiers at repeaterstations along the line, in which case the oscillatory circuit maycomprise a closed loop including these amplifiers in tandem relation andfrequency selective means for inhibiting oscillations around the loopexcept at the desired pilot frequency. Inasmuch as the power output fromeach amplifier is made constant, the difference between the power outputof one amplifier and the power output of the succeeding amplifierremains invariable, at zero, for example, and wholly independent ofchanges in the attenuation of the interposed section of transmissionline. In other words, the transmission equivalent of the repeatersection is independent of line attenuation. At 0 the same time thesignal level at each amplifier is permitted to follow in normal mannerthe variations in the level of signals applied to the system, despitethe constant output power control of the amplifier, for as the level ofapplied signals changes, the level of the pilot frequency oscillationsautomatically changes an equal amount in the opposite sense, so that therepeater gain control is not aware of any change in signal level, perse.

In one aspect, the invention involves a principle of operation of anoscillatory system having an amplitude-limiting or gain-changing controlelement responsive to the currents flowing in the systern, viz., thatwhen signals or other waves are applied to the control element theamplitude of oscillations automatically readjusts itself to such a valuethat the applied wave and the oscillations of new amplitude have acombined effect upon the 40 control element that is the same as theoscillations alone had before the application of the other waves. Inanother aspect, the invention may be said to provide a pilot wave theintensity of which varies in such manner in relation to the variationsin the intensity of the signal waves that the average intensity of pilotwave and signals combined is substantially constant.

The nature of the present invention and its various objects, featuresand advantages will appear more fully in the following detaileddescription of typical embodiments illustrated in the accompanyingdrawing:

Fig. 1 of the drawing illustrates a four-wire transmission systemutilizing an embodiment of the invention for automatic transmissionregulation; and

Figs. 2 and 3 illustrate modifications thereof.

Referring more particularly now to Fig. 1, there is shown a four-wiretransmission system in which signals are transmitted over one pair ofconductors in the W-E direction from source S to receiver R, and in theEW direction over another pair of conductors from source S to receiverR. The signals to be transmitted may be voice frequency telephonesignals, television signals or of any other desired character, but forpurposes of further exposition of the invention it will be assumed thatthe system is a multiplex carrier telephone system and that the twosources are terminal circuits of the system providing, for specificexample, twelve carrier telephone channels spaced apart in the frequencyrange from 12 to 60 kilocycles per second. The two receivers are thenthe Corresponding receiving terminal circuits of the system, and eachmay be connected in the usual manner through hybrid coils H with itsassociated transmitting terminal to provide twelve two-way voicefrequency circuits. The total wave output of the source, S or S, will bereferred to as constituting the signal.

The wire lines constituting the transmission medium may be of anysuitable form, such as open-wire lines, coaxial conductor pairs orshielded pairs in the same or different multipair lead sheathed cables.In any case the lines are subject to one or more influences, such astemperature changes, which cause the line attenuation to fluctuate. Anobject of the invention, as indicated hereinbefore, is to minimize theeffect of such fluctuations on the signal intensity and to maintain thetransmission equivalent substantially constant for both directions oftransmission.

One or more repeater stations is or may be included in the four-wiresystem, and in Fig. 1 there is indicated one such station comprising asignal amplifier i in the WE line and a signal amplifier 3 in the EWline. Similar amplifiers 2 and 4 are provided at the receiving terminalsfor amplifying the signals received from the respective last sections ofthe two lines. Each of these four amplifiers is of the stabilizednegative feedback type, in which the amplificationfrequencycharacteristic is essentially determined by the attenuation-frequencycharacteristic of its beta or feedback circuit. Each, further, comprisesthe usual beta circuit network 2 which compensates for the normalattenuation-frequency characteristic of the preceding section oftransmission line, and a thermistor 2|, also in the beta circuit, whichis so proportioned and arranged in the circuit in known manner that thewave power output of the amplifier remains substantially constantdespite a fairly wide range of variation of power applied to the inputterminals of the amplifier. An amplifier of this general description isdisclosed in, and described with reference to 7 of an application forLetters Patent, Serial No. 114 390 filed by H. S. Black, December 5,1936 (U. S. Patent No. 2,209,955, issued August 6, 1940), and in anapplication of J. H. Bollman, Serial No. 280,269, filed of even dateherewith. It will be understood that should the wave power output ofsuch an amplifier tend to increase, for example, the current traversingthe thermistor and therefore its temperature likewise tend to increase,with the result that the beta circuit attenuation and the overall gainof the amplifier tend to decrease, thereby reducing the wave poweroutput to its original value. The beta circuit and more particularly thethermistor 2|, can be so proportioned that the response to changes inwave power input is fairly rapid, rapid enough, for example, that inputpower fluctuations of syllabic frequency do not cause correspondingfluctuations of output power.

In the WE direction of transmission, the signal from the source S isfirst amplified in the transmitting terminal amplifier 5, the gain ofwhich may be comparable with that of the repeater amplifiers,transmitted over the first line section L1 to repeater I, thence overline section L2 to the receiving terminal amplifier 2 and the receivingterminal circuits R. In the E-W direction of transmission, the signalfrom source S passes through the transmitting terminal amplifier 6, linesection L3, repeater 3, line section Li and receiving terminal amplifier4 to the receiving terminal circuits R.

Connected in series between the output circuit of receiving terminalamplifier 2 and the input circuit of the transmitting terminal amplifier6 in the oppositely-directed line, is a narrow-band filter l and aresistor 9. filter 8 and a resistor iii are similarly connected inseries between the input terminals of amplifier 5 and the outputterminals of amplifier 4. The resistors may be considered asrepresentative of resistance pads or other attenuating means. The twofilters are tuned alike to pass a substantially single frequency f1lying in a portion of the frequency range not occupied by the signalingchannels. This frequency may lie between the channels or above or belowthe 12 to 60-kilocycle range specified, but it is preferably such thatthe attenuation at the frequency selected varies in substantially thesame manner as the average attenuation over the signaling range. Therecan now be identified a transmission loop comprising in tandem relationthe two filters and their associated resistors, the six amplifiers ofthe system and the interconnecting line sections. Sustained oscillationsat the pilot frequency f1 to which the filters l and 8 are tuned willappear in this loop if there is a net transmission gain around it, thatis, if the amplification afiorded by the six amplifiers exceeds theattenuation offered by the transmission lines and the resistors 9 andi8, and if the total phase shift around the loop is zero or an integralmultiple of 211".

Supposing that there are no signals being transmitted over the system,the pilot frequency oscillations will tend to increase indefinitely inamplitude. Opposing this tendency, however, is the fact that each of theamplifiers I, 2, 3 and 4 is designed to have a constant, predeterminedpower output and any tendency of the power output to exceed thepredetermined value results in such automatic reduction in the gain ofthe amplifier that the power output is restored to or maintained at thatpredetermined value. The intensity of oscillation increases, therefore,to the point where the concurrently reduced gain of the fourself-regulating amplifiers is equal to the losses in the transmissionloop, that is, to the point where the net loop gain is zero at the pilotfrequency f1. Resistors 9 and [0 can be designed or adjusted to fix theintensity of the oscillations at any predetermined level under theconditions assumed.

Consider next what takes place when a signal of given intensity from saysource S is applied to the system. This signal, appearing at each ofamplifiers I and 2, tends to increase the total Another narrow-bandpower output of each amplifier, inasmuch as its power is added to thatof the pilot'frequency oscillations. This tends in turn to effectautomatically a reduction in the gain of the amplifier and a reductionin the intensity of the pilot frequency oscillations. An equilibriumcondition is finally reached in which at each of the selfregulatingamplifiers I and 2 the total power output of signal and pilot wavecombined is just equal to the power output obtaining before the signalappeared in the system, and the net gain around the transmission loop atthe pilot frequency i1 is again zero.

It has been implicitly assumed in, the foregoing description that thegain of amplifiers 3 and 4 does not change with changes in the intensityof the pilot frequency oscillations. The assumed condition can bepractically realized, however, by making resistor 9 of high resistanceand resistor II] of low resistance so that the pilot frequencyoscillations applied to amplifier 6 are of such small intensity as tohave substantially no gain-controlling effect at amplifiers 3 and 4.

The intensity of the signal varies continually, for it depends on thenumber of carrier channels in use at any given time and on the varyingspeech level of each talkers voice, and there is also a variation inintensity from syllable to syllable. Superimposed on these normal,initial intensity variations are the variations due to the slowlychanging attenuation of the transmission line carrying the signal.Regardless of the cause of a given change in signal intensity, however,there is an equal and opposite change in the intensity of the pilotfrequency oscillations so that the intensity of signal and pilot wavecombined at the output of each of the self-regulating amplifiers ismaintained substantially constant. Although it is important that thecombined intensity remain constant, it is equally important that thesignal arrive at the receiving terminal intact in so far as its initial,normal intensity variations are concerned, or, in other words, that theintensity of the signal bear the same ratio to the intensity of thepilot wave at the receiver end of the transmission line as obtains atthe transmitter end of the line. To satisfy this latter condition it canbe shown that it is necessary only that the transmission equivalent ofthe portion of the pilot wave loop extending from the output circuit ofthe amplifier 2 to the input circuit of the amplifier 5 be maintainedsubstantially constant. That it does remain substantially constant isinsured by automatic gain regulation provided for the EW transmissionline as described hereinafter.

It may be observed that the intensity of the pilot wave at the input ofamplifier 5 where it is introduced into the WE line, fluctuates only asthe initial intensity of the signal fluctuates and that the pilot wavefluctuations are of such sense and magnitude that the intensity of pilotwave and signal combined is substantially constant at this point. It maybe noted too that the system embodies the general principle statedhereinbefore inasmuch as in the oscillatory loop circuit the sum of theintroduced signal currents and the selfoscillations of pilot frequencyhas the same effect on the gain-controlling thermistors as the selfoscillations alone had before the introduction of the signal currents.

To effect automatic regulation of the gain of amplifiers 3 and 4 in theE-W transmission line another loop transmission path tuned to adifferent pilot frequency I2 is provided. This second path comprises thesix amplifiers and their interconnecting line sections, a filter II andresistor I3 in series with each other and in parallel with the seriescombination of filter l and resistor 9, and a. filter I2 and resistor I4similarly connected in parallel with filter 8 and resistor III. Thefilters II and I2 are narrow-band filters tuned to pass a frequency insuitable for the second pilot wave.

Oscillations established in this second path operate on the thermistorsin amplifiers 3 and 4 to control the gain thereof in precisely the samemanner as the oscillations in the first-described loop path control thegain of amplifiers I and 2. Resistor I4 is made of high resistance andresistor I3 of low resistance so as substantially to eliminate theeffect of the second pilot wave on the gain of amplifiers I and 2.

To reduce interaction between the two gain control systems embodied inFig. l, the circuit may be modified as shown in Fig. 2. As compared withFig. 1, the modification consists in replacing amplifiers 5 and 6 withself-regulating amplifiers I 5 and I6, respectively, of the same typeindicated for the repeaters; in excluding amplifier I6 from the firsttransmission loop; and in excluding amplifier I5 from the secondtransmission loop. The changes in the transmission loops consist inconnecting filter I and resistor 9 to the output circuit rather than tothe input circuit of amplifier I6, and in connecting filter I2 andresistor I4 to the output circuit rather than to the input circuit ofamplifier I5. In the circuit as thus modified, changes in the intensityof either of the pilot waves have less effect on the gain of theamplifiers intended to be controlled by the other pilot wave. Thus, inthe transmission loop comprising filters 1 and 8 the loop gain iscontrolled primarily by the three thermistors in amplifiers I5, I and 2,whereas only two of these thermistors, viz., those in amplifiers I and2, have any effect on the gain of the other transmission loop whichcomprises filters II and I2. Similarly in the latter transmission loopthe loop gain is controlled primarily by the self-regulating amplifiersIE, 3 and 4, and the pilot wave in that loop operates on only two of theself-regulating amplifiers, viz., I and 2, in the other loop.

Fig. 3 shows another modification of the system shown in Fig. 1, which,like that of Fig. 2, is designed to reduce interaction between the gaincontrol circuit for EW transmission and the gain control circuit for W-Etransmission. As compared with Fig. 1 the modification consists in theprovision of self-regulating amplifiers I! and I8, of the kind specifiedfor the repeaters, which in circuit sequence immediately follow thereceiving terminal amplifiers 2 and 4, respectively, and inreconstructing the two transmission loops so that each embraces the oneamplifier, I! or I8, that is included in the signaling circuit that loopis designed to regulate. Thus, amplifier I! in the W-E circuit is;included in the transmission loop comprising filters I and 8 butamplifier I8 is excluded, and amplifier I8 is included in thetransmission loop comprising filters I I and I2 but amplifier I7 isexcluded. The net result, as in Fig. 2, is that each of the pilot wavescontrols the gain of a greater number of the amplifiers in itsrespective transmission loop than are controlled by the pilot wave inthe other loop.

Whereas in the foregoing illustrative examples of practice in accordancewith the invention the gain-controlling elements are responsive to thetotal power applied to them, it should be apparent that oth r types ofgain-controlling elements responding to peak values of current orvoltage or in general utilizing some measure of wave intensity otherthan power can be used in the same way to perform the desired regulatingfunctions. The invention is not limited to such details as these and itembraces any modifications that come within the spirit and scope of theappended claims.

What is claimed is:

l. A loop transmission circuit comprising means for maintainingsustained oscillations therein. means in said circuit for controllingthe transmisequivalent thereof, a signal source, and means for applyingboth the signals from said source and said oscillations to saidcontrolling means, whereby the said transmission equivalent is subjectto joint control by sa d signals and said oscillations.

2. A combination in accordance with claim 1 in which said controllingmeans operates to maintain the intensity of said signals and saidoscillations combined substantially constant.

3. A transmission system comprising asignal source, a plurality ofamplifiers geographically spaced apart for successively amplifying thesignals from said source, a transmission loop comprising all of saidamplifiers, said transmission loop being oscillatory at a frequency notoccupied by said signals, and means at each of a plurality of saidamplifiers for controlling the gain thereof, said controlling meansbeing responsive to said signals and oscillations of said frequencycombined.

4. In combination, an oscillatory transmission loop, an amplifierincluded therein, a thermistor traverse by oscillations in said loop forcontrolling the gain of said amplifier, means for applying signals tosaid amplifier for amplification therein, and means for applying theamplified signals in -fixed proportion to said thermistor for concurrentcontrol of the gain of said amplifier.

5. In combination, a negative feedback amplifier including again-controlling directly-heated thermistor in the beta circuit thereof,said thermistor being so proportioned and arranged as to maintain thetotal wave power output of said amplifier substantially constant, meansfor applying signals to said amplifier for amplification therein, and acircuit including said amplifier in tamden relation therein forgenerating oscillations, whereby said oscillations are modulated by saidsignals.

6. In combination, a signal source, a signal receiver and means forimpressing the signals from said source on a transmission medium thatvariablv attenuates said signals in their passage from said source tosaid receiver, a transmission loop comprisiig said medium and saidreceiver and means for establishing oscillations therein, and means atsaid receiver for maintaining the intensity of said signals andoscillations combined substantially constant.

'7. In a signaling system, a signal source, a device for amplifying thesignals from said source, said device having an input circuit and anoutput circuit, a first loop circuit connecting said input and outputcircuits and constituting a negative feed circuit for the frequencyrange occupied by sa 6. signals, a second loop circuit connecting inwhich said responsive means comprises a thermistor.

9. In combination, an amplifier, means for applying signals to beamplified thereto, transmission means connecting the input and outputcircuits of said amplifier to form a self-oscillatory loop tuned to passat least one substantially single frequency, and means responsive to thetotal wave output of said amplifier including oscillations generated insaid loop for controlling the gain of said amplifier.

10. A combination in accordance with claim 9 in which saidgain-controlling means operates to maintain the total wave power outputof said amplifier substantially constant.

11. In a signaling system, a signal source, a transmission line subjectto variations in attenuation for transmitting the signal from saidsource, an amplifier in said transmission line, and means completing atransmission loop comprising said I amplifier and said line, theattenuation-frequency characteristic of said loop being such thatoscillations of substantially single frequency occur therein, and saidamplifier being so self-adjusting as to maintain the total waveintensity of said signals and oscillations combined substantiallyconstant at its output terminals.

12. In combination, a signal source, means for amplifying the signalsfrom said source, and a long transmission line for conveying theamplified slgnals from said means, and an oscillatory transmission loopcomprising said amplifying means, a substantial portion of said linesubject to variations in attenuation and filtering means adapted to passa substantially single frequency comparable with the frequency of saidsignals, said amplifying means being so self-adjusting under the controlof its total wave output as to maintain said total wave output at asubstantially constant intensity level.

13. In a signaling system, a long transmission line subject tovariations in attenuation, a signal source connected to one end thereof,one or more signal amplifiers disposed in said line and geographicallydistant from said source, at least one of said amplifiers being soself-adjusting that the total wave power output therefrom issubstantially constant despite wide variations in the wave power inputthereto, a circuit extending from the output circuit of one of saidself-adjusting amplifiers to a point of said line in the vicinity ofsaid source so that a loop transmission path is formed comprising saidline and said last-mentioned amplifier, the transmission characteristicsof said loop path being such that oscillations tend to occur at asubstantially single frequency.

14. A system for the transmission of signals beween geographicallyseparated points comprising two transmission lines connecting saidpoints and individual to the two directions of signal transmissiontherebetween, a signal amplifier in each of said lines at the receivingpoint thereof, means at each of said points connecting the outputcircult of one of said amplifiers to the other line, said means having asubstantially fixed attenuaon-frequency characteristic such that twoloop transmission channels tuned to different and substantially singlefrequencies are provided. each of said loops including said transmissionlines and amplifiers, and each of said amplifiers being soelf-regulating as to gain that the total wave power output therefrom ismaintained substanially constant.

15. A four-wire signal transmission system comprising a pair of linesfor transmission in the respective signaling directions, each of saidlines being subject to variations in attenuation due to changes intemperature and the like, at least one signal repeater intermediate theterminals of each of said lines and a signal amplifier at the receivingterminal of each of said lines, each of said repeaters and saidamplifiers being so self-regulating as to gain that the total wave poweroutput thereof remains substantially constant for a wide range of wavepower input thereto, frequency selective means connecting each pair ofadjacent terminals of said lines and comprising two transmission loopseach including both of said lines, said repeaters and said amplifiers,said loops being tuned to pass and to oscillate continuously at mutuallydifferent substantially single frequencies, whereby the gain of each ofsaid repeaters and amplifiers is subject to the joint control of thesignals and oscillations traversing it and the transmission equivalentof each of said lines is maintained substantially constant.

16. A system in accordance with claim 15 comprising means forsubstantially attenuating both of said oscillations at respectivelydiiferent terminals of said system, whereby in each of said lines theintensity of a respective one of said oscillations is so reduced as tohave small effect on the transmission equivalent of that line.

17. A system in accordance with claim 15 comprising at the sendingterminal of each of said lines a signal amplifier adapted forsubstantially constant wave power output, each of said amplifiers beingincluded in only one of said loops.

18. A system in accordance with claim 15 comprising at each of saidreceiving terminals a second signal amplifier adapted for substantiallyconstant wave power output, each of said second signal amplifiers beingincluded in only one of said loops.

19. A system in accordance with claim 15 in which each of said loopsincludes a greater number of the signal-amplifying, constant poweroutput elements in one of said lines than are included in the other ofsaid loops.

20. In a signaling system, a transmission link subject to randomvariations in transmission equivalent, a closed oscillating loop systemincluding said transmission link so that the intensity of theoscillations in said loop system tends to vary with said variations intransmission equivalent, and means controlled by said oscillations forvarying the transmission equivalent of said link in opposite sense tosaid random variations.

21. In a signaling system, a transmission link subject to randomvariations in a transmission characteristic thereof that tends toproduce corresponding variations in a parameter of signals transmittedthrough said link, a closed oscillatory loop system including saidtransmission link in such manner that said parameter of the oscillationsin said loop system tends to vary with said random variations, and meanscontrolled by said oscillations for varying the said transmissioncharacteristic of said link equally and oppositely to said randomvariations.

22. In combination, an electric wave amplifier, means for applyingsignals of variable average intensity to the input of said amplifier,means for applying concurrently to the input of said amplifiersubstantially single-frequency oscillations, and amplifier gaincontrolling means responsive to the joint control of said signals andsaid oscillations for maintaining the average wave power output ofoscillations and signals combined substantially constant.

23. In combination, a signal amplifier and gaincontrolling meanstherefor, said means comprising a loop circuit including said amplifierfor the generation of oscillations, an impedance elements variable underthe joint control of said oscillations and signals to maintain constantthe total average intensity of said oscillations and signals combined,and means external of said amplifier for varying the attenuation in saidloop circuit and thereby controlling the gain of said amplifier.

24. In combination, a source of signals of normally varying averageintensity, an amplifier for amplifying the signals from said source, awave transmission loop including at least a portion of said amplifierfor the generation and loop transmission of oscillations lying outsidethe frequency range occupied by said signals, and means for varying theintensity of said oscillations in equal and opposite relation to thevariations in the said average intensity of said signals.

25. In combination, a source of signals having normal initial variationsin average intensity, a device for amplifying the signals from saidsource, said amplifier having a feedback path for the gain-reducingfeedback of said signals, a loop transmission circuit including at leasta part of said amplifier for the generation of substantiallysingle-frequency oscillations, and an impedance under the joint controlof said signals and oscillations for maintaining the total output ofsaid amplifier, comprising said oscillations and signals, substantiallyconstant.

26. A combination in accordance with claim 25 in which said impedancecomprises a thermistor heated by said oscillations and signals insubstantially constant proportion.

ROY W. CHESNUT.

